High spatial resolution performance of a triple Fabry–Pérot filtergraph

We investigate the consequences of monochro- matic pupil apodisation in a ltergraph based on Fabry{ P erot etalons in a telecentric mount, an eect which was discovered by Beckers (1998a). The pupil apodisa- tion gives rise to systematic changes of the point spread function in the presence of strong spectral gradients, like in the wings of a Fraunhofer line. These changes are likely to produce spurious velocity signals in Dopplergrams gen- erated from high spatial resolution ltergrams of the solar surface. We include in our analysis for the rst time the ef- fects of optical phase changes on pupil apodisation which has the tendency to enhance velocity errors. We study the resolution and the velocity errors for the three{etalon ltergraph TESOS (Kentischer et al. 1998) at the Vacuum Tower Telescope on Tenerife. Peak{to{valley velocity errors for solar structure with sizes of 2 arcsec and less and a contrast of 15% amount to less than 50 m/s for the low resolution mode and to less than 10 m/s for the high resolution mode of TESOS in the worst case. We conclude that the performance of TESOS is not severely compromised by pupil apodisation.